When animals practice monogamy, we can generally tell right away. After all, couples like to hang out together, whether they're humans or squirrels or hawks. But one fish keeps its monogamy so top-secret that researchers actually called their pairings "invisible."

The cichlid fish species Xenotilapia rotundiventralis is only 5 centimeters long and lives in vast schools of over 2,500. That's the sort of vast conglomeration of fish that's practically impossible for our eyes to comprehend, let alone pick out which fish have paired off together in reproductive couples.

But these fish want to make it even harder for us - they're the only known species that practices social monogamy without bothering with the whole physical proximity thing. Basically, a fish in one of these schools is no more likely to spend time with her husband than she is any other fish. As such, picking out the couples in these schools is an almost entirely random process.

Now, researchers from Japan's Kyoto University and Osaka City University have figured out how to identify which fish are forming pair bonds, even if we can't see them spending extra time with each other. The secret all comes down to how the fish care for their young. These fish use a process known as mouthbrooding, wherein the mothers first hold their young in their mouths and then transfer them to males. These males, it turns out, are almost always the genetic fathers, which means the two fish stay in a monogamous relationship at least until their young are grown.

The researchers explain:

The Lake Tanganyika cichlid fish Xenotilapia rotundiventralis forms schools that consist of mouthbrooding and non-brooding adults in mid-water, and visible pairs are not recognized. A previous study suggested that mouthbrooding females transfer fractions of the young to males when the young become large. However, it remains a mystery whether the mating pairs maintain pair bonds so that the females can transfer the young to their mates.

To answer this question, we conducted a parentage analysis using 10 microsatellite markers. The analysis showed that the mouthbrooding adults were most likely genetic fathers and mothers of the young in their mouths. This finding suggests that the female-to-male shift of young takes place between mating partners, and thus the mating pairs maintain pair bonds at least until the shift of young. The present study is the first to detect pair bonds in animals in which physical proximity has not been observed.